1. Field of Art
The present invention relates to a optical disk type recording system wherein informations are written in a recording surface of a optical disk by means of focussed light beams and the informations are read out by means of light beams reflected by the recording surface of the disk. More particularly, it relates to an improved focussing error detection device for use in such a optical disk type recording system.
2. Prior Art
There have been known a optical disk recording system wherein pits are formed by focussing a light beam, such as a laser beam, on a recording surface of a optical disk to write the informations in the disk, a optical disk reproducing system for reading out the information by means of reflected light beams from the recording surface or the surface on which the informations have been written, and a optical disk recording-and-reproducing system for writing-in and for reading-out the informations. In such a system, the area of the recording surface impinged by the light beam becomes too large to result in failure to secure stable recording and reproducing operations, unless the light beam is precisely focussed on the recording surface.
In order to secure the stable operations, focus servo means is indispensable to detect the error in the distance between the recording surface and the objective lens means continuously to determine the focussing error (hereinafter referred to as FER) and to maintain the distance from the recording surface to the objective lens means at a constant distance.
The known methods for detecting the FER include the method utilizing the astigmatic aberration, the Foucault's method and the critical angle method. In the method utilizing the astigmatic aberration, a cylindrical lens is used to differentiate the longitudinal focal length along the transverse direction of the reflected light and the FER is detected by the change in output of the quarterly splitted light detector disposed in the vicinity of the focal plane. In the Foucault's method, a prism is disposed such that one edge thereof extends along the focal plane of the reflected light to split the reflected light into two light beams and the difference in outputs between two sets of dually splitted photo detectors for detecting the dislocations of respective divided light beams is compared to determine the FER. In the critical angle method, the change in intensity distribution of the reflected light upon reflection by the critical angle prism is detected by a dually splitted photo detector to determine the FER, while making use of the phenomenon that the light reflected by the recording surface forms either of the diverging beam or converging beam upon passage through an objective lens depending on the change in distance between the recording surface of the disk and the objective lens.
As described above, in all of the known methods, photo detectors for detecting quarterly or dually splitted light beams are used and the focussing errors are determined by detecting the difference between the outputs of the light detecting elements of each photo detector. However, such a quarterly or dually splitted photo detector has inevitably an insensitive zone so that it cannot detect the difference whithin an region of the recording surface corresponding to the insensitive zone. As a result, the sensitivity of detection by the use of such a detector cannot be improved over a limited level.
Another disadvantages of the known methods are that the photo detectors and the prism must be positioned precisely to necessitate cumbersome adjustment in positioning. Moreover, in the method utilizing the astigmatic aberration and in the Foucault's method, the light reflected by the recording surface must be converged to result in that a certain space is occupied for the light path of the converging light to limit the free utilization of the space.